The transition duct (transition member) 1 of a gas turbine engine 2 (FIG. 6) is a complex and critical component. The transition duct 1 serves multiple functions, the primary function being to duct hot combustion gas from the outlet of a combustor 3 to an inlet of a turbine 4 within the engine casing 5. The transition duct also serves to form a pressure barrier between compressor discharge air 6 and the hot combustion gas 7. The transition duct is a contoured body required to have a generally cylindrical geometry at its inlet for mating with the combustor outlet and a generally rectangular geometry at its exit for mating with an arcuate portion of the turbine inlet nozzle. The high temperature of the combustion gas imparts a high thermal load on the transition member and thus the transition ducts of modern gas turbine engines are typically actively cooled. Transition members may be cooled by effusion cooling, wherein small holes formed in the duct wall allow a flow of compressor discharge air to leak into the hot interior of the transition member, thereby creating a boundary layer of relatively cooler air between the wall and the combustion gas. Other designs may utilize a closed or regenerative cooling scheme wherein a cooling fluid such as steam, air or liquid is directed through cooling channels formed in the transition member wall. One such prior art steam-cooled transition duct 10 is illustrated in FIG. 1, where it can be seen that the generally circular inlet end 12 converts to a generally rectangular outlet end 14 along the length of flow of the combustion gas carried within the transition member 10. The axis of flow of the combustion gas is also curved as the combustion gas flow is redirected to be parallel to an axis of rotation of the turbine shaft (not shown). The corners of the transition duct 10 tend to be highly stressed, particularly the corners 16 proximate the outlet end 14 due to the combination of the corner geometry and a higher gas velocity due to a reducing duct flow area and turning effects. One prior art approach to address these highly stressed regions is the use of a highly engineered and specific duct profile, such as is described in U.S. Pat. No. 6,644,032. Such approaches may not be desired because they reduce the available design options.
The manufacturing process used to form the component further exacerbates the stress concentration in the corners of the transition duct 10. Prior art transition members are formed by welding together a plurality of panels that have been pre-formed to a desired curved shape. FIG. 2 is a cross-sectional view of the prior art steam-cooled transition duct 10 illustrating how the component is formed by joining four individual panels 18, 20, 22, 24 with respective welds 26. The welds 26 are located in the corners in order to minimize forming strains and wall thinning/thickening when the panels are bent. However, the placement of the welds 26 in the corners precludes the location of cooling channels 28 in the corners, and adjacent channels must be spaced far enough from the welds 26 to ensure that their functionality is not compromised during welding. The corners are thus poorly cooled.